Periodontitis is an inflammatory disease that causes destruction of the tooth-supporting tissues and may adversely affect systemic health. The proper function of homeostatic mechanisms is essential for protection against unwarranted inflammatory tissue damage. The endothelial cell-secreted protein Del1 (encoded by the EDIL3 gene) acts homeostatically in vivo to regulate neutrophil transmigration and local inflammation in the periodontal tissue. However, Del1 expression is diminished in old age or by certain inflammatory stimuli. Preliminary studies also suggest that Del1 is additionally expressed by and regulates osteoclasts. Therefore, Del1 can potentially act as a gatekeeper of inflammation and may have important therapeutic implications. The overall objective of this application is to understand how Del1 expression is regulated at the molecular signaling level, define novel Del1 functions, and identify key functional sites within the Del1 polypeptide that could be exploited as anti-inflammatory drugs. The proposed approach is based on in vitro and in vivo experimental systems in mice (wild-type and Del1-deficient) and non-human primates and in vitro human assays for transmigration, osteoclastogenesis, and Del1 regulation. The overarching hypothesis of this proposal is that Del1 is a functionally versatile molecule which acts homeostatically to regulate critical upstream and downstream events that lead to inflammatory bone loss, and, therefore, has therapeutic potential for the treatment of periodontitis. Four Specific Aims are proposed. Aim 1 will determine the role and mechanism(s) of Del1 in the differentiation and function of osteoclasts. Aim 2 involves the dissection of signaling pathway(s) regulating Del1 expression. Aim 3 involves the functional mapping of domains of Del1 essential for regulating neutrophil transmigration or osteoclastogenesis. Aim 4 will determine the efficacy of local treatment with Del1 (or domains thereof) in experimental periodontitis in non-human primates. The long- term goal of this project is to establish Del1 (or Del1-derived segments) as an effective adjunctive treatment for human periodontitis. As the protective role of Del1 is suspected in additional inflammatory conditions, this proposal may have implications beyond the treatment of periodontitis. Importantly, since Del1 is an endogenous molecule the expression of which is diminished in inflammatory disorders, the local administration of recombinant Del1 (or segments thereof) in inflammatory conditions is unlikely to involve safety issues, but rather should restore tissue homeostasis.